It is well known in the art that magnetic sensors can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetic sensor is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative, and in close proximity, to the magnetic sensor, such as a toothed wheel, produces a varying magnetic flux density through the magnetic sensor.
FIG. 1A is an example of a prior art magnetic sensor 50, wherein the magnetic sensor (MS) element 10 is mounted on the bottom surface 12 of a permanent magnet (bias magnet) 14 magnetized in a direction 16 perpendicular to the direction of motion 18 of target wheel 20 having teeth 22 and slots 24.
FIG. 1B is a plot 52 of the magnetic flux density detected by the prior art magnetic sensor 50 of FIG. 1A as the target wheel 20 passes the magnetic sensor. The larger magnetic flux density 22′ represents the passage of a tooth 22 past the sensor 50, whereas the smaller magnetic flux density 24′ represents the passage of a slot 24 past the sensor.
The resolution of magnetic sensor 50 is related to the number of teeth 22 of target wheel 20. Increased resolution is achieved in a prior art magnetic sensor in one of three ways. One possible way consists of increasing the number of teeth 22. This approach is limited at some point, because the teeth 22 and slots 24 must be of a sufficient physical size to affect the magnetic field in the MS element 10, and, in some applications, the number of teeth 22 is fixed by external constraints, for instance, when target wheel 20 is a gear used for both mechanical advantage and for position sensing. The number of teeth 22, in such a case, may not be sufficient to provide the desired resolution. Another way consists of placing several MS elements 10 on the magnet surface 12. This, however, adds cost. Yet another way consists of using a phase-lock loop (PLL). This approach, however, suffers from errors during accelerations and decelerations. Finally, some applications, for instance spark-plug firing control, could use very fine position information, possibly within plus or minus one-half degree in that example, yet correspondingly fine resolution sensors (360 pulses per revolution in that example) are generally too costly.
Accordingly, what is needed in the art is a more robust magnetic sensor design enabling the resolution of the magnetic sensor to be increased.